Polypropylene is a versatile and widely used thermoplastic. Many of its uses are in applications that take advantage of its excellent physical properties in molded shapes. However, while polypropylene has outstanding stiffness (i.e., tensile and flex properties), it often fails in a brittle mode on impact, especially at sub-ambient temperatures, and so for many applications it is desirable to increase its toughness. At the same time, it is often desirable to maintain a minimum level of stiffness, such as for automotive parts, which frequently need to be both stiff (high flexural modulus) and tough (high energy-to-break under impact and/or tensile deformation).
To mitigate the brittle nature of polypropylene, and therefore improve its impact resistance and tensile toughness, softer (less crystalline) polymers are often added to polypropylene. These softer polymers include ethylene elastomers such as ethylene-propylene rubber (“EPR”) or ethylene-propylene-diene monomer rubber (“EPDM”), which are generally characterized by from about 0 to about 10% crystallinity and a density below 0.86 g/cm3. Ethylene/α-olefin copolymers involving higher α-olefins such as butene, hexene, octene and the like have been also been used either in conjunction with, or in place of, EPR or EPDM. These ethylene copolymers—also known as “plastomers”—generally have a crystallinity ranging from about 5 to about 40%, high extractability, and a density in the range of 0.86 to 0.92 g/cm3. Ethylene-butene plastomers, in particular, have been added to impact modify polypropylene. For example, the advantageous use of ethylene-butene plastomers having an upper density limit of 0.910 g/cm3 are described in “Impact Modification of Polypropylene with Exact™ Plastomers” (Society of Plastics Engineers, 1994). However, soft polymeric impact modifiers inevitably tend to reduce the stiffness, or rigidity, of the polypropylene polymers that they toughen. An additional drawback of PP/EPR or PP/EPDM or PP/plastomer blends is cost. Typically, EPR, EPDM, and plastomer are each more expensive than polypropylene.
Therefore, there is a need for a polypropylene blend that improves the stiffness-toughness balance of the blend as compared to previously disclosed polypropylene blends. There is also a need for a polypropylene blend that retains or improves the stiffness-toughness balance of the blend as compared to previously disclosed polypropylene blends, and costs less than previously disclosed polypropylene blends. These blends will be especially useful in molded parts, and more particularly useful in interior and exterior parts of vehicles, such as trucks, passenger automobiles, boats and the like.